This is somewhat difficult for me because I mostly agree for Gray, but there are some very serious problems with this book. First off, Gray's analysis of communism and nazism is cursory and repetitive. It has been done better by many, by Voegelin (whom Gray cites once), Kolakowski (whom Gray also only cites once) and probably by Cohn (whom Gray seems to have gathered his argument from, since he doesn't claim to have taken it from Voegelin or Kolakowski). I must take issue of his dismissing of Arendt (particularly two very disparate books of hers in about two paragraphs) as I don't think he takes her seriously enough.
Things really improve when he moves on to his critique of neo-conservatism and his critiques of Bush and Thatcher (though the critique of Thatcher is half-assed). His points about neoliberalism and neoconservatism are well taken and I am annoyed that he made them because I wanted to write a book about it. Boo. Anyway, he's mostly right and this is the strength of the book. Honestly, in recommendation, I say read those chapters and don't worry too much about the initial chapters. If you really want to understand how communism (and to a lesser extent nazism) is a religion, you should read Kolakowski, not Gray.
So things are going well, but then he says "The faith in Utopia, which killed so many in the centuries following the French Revolution, is dead." I thought he was joking. That's the kind of thing I would have written in a book like this only to say "Fooled You! Hahaha" (booming in Darth Helmet voice). He criticizes people for proclaiming the end of history and then proclaims the end of an idea. Has he not darkened the door of a university in a while or what? Utopianism is alive and well, not just in the remains of neo-conservatism (which, I hate to break it to him, is alive and kicking in the United States) but also in the numerous emancipatory "post-" ideologies. It is disguised better, but it's still there. In this same chapter he also weakly admits that conservatism can be utopianism, but he does not spend time critiquing it, like he was willing to criticize socialism and liberalism. Finally, his conclusion that Realism (capital r) is the only solution is a little bizare (partly because one of the guys he cites as a realist, Hedley Bull, is hardly a realist is the conventional sense). Are we talking about international relations or internal politics? He isn't sure. In the final chapter he lumps in the liberal tradition of political theory with the liberal tradition in international relations, which are two very different things. Realism may be a reasonable critique of the liberal tradition of international relations, but it is not even a critique of the liberal tradition of political theory.
I could go on and on and on. Chapters 3-5 are very worth your time, if repetitive. But the rest of the book has some major issues.